n 1 : a horny sheath of thickened and condensed epithelial stratum lucidum that grows out from a vascular matrix of dermis and protects the upper surface of the end of each finger and toe of humans and most other primates and that is strictly homologous with the hoof or claw of other mammals from which it differs chiefly in shape and size - called also nail plate 2 : a structure (as a claw) that terminates a digit and corresponds to a nail 3 : a rod (as of metal) used to fix the parts of a broken bone in normal relation <a medullary > .